Calculation examples:
1. Marine water
Marine water has about 32 to 35 gramms of salt per each liter water. This example works with an average value of 33 gramms per liter.
Place a mangrove in a pot with 3.8 liter volume.

3.8*33 = 125 gramms salt is required

2. Brackish water
Brackish water has about 10 to 20 gramms of salt in each liter of water. This example works with an average value of 15 gramms per liter.
Someone wants to create a mangrove basin which is 120x40x50cm (50cm is the height). How many mangroves will be placed in this mangrove basin is not important for our calculation. The mangrove basin will be filled with 15cm of ground substrate and the water level will end at 25cm.

Note:
The salinity (amount of salt in a liquid) increases and decreases with the amount of water. If the water is used by the plants or evaporated the salinity will increase, immediately after watering the salinity will decrease. Mangroves are perfectly used to fluctuations of the salinity.

Attention:
Salt does not disappear and remains in the water. Therefore just add salt only once. Just add new salt when you have to adjust the salinity, for instance when you put the mangrove into a bigger pot to grow in.